1. Field of the Invention
This invention generally relates to an optical information reproducing method and an optical information reproducing apparatus. This invention particularly relates to a method and an apparatus for optically reproducing, from an optical information recording medium, information which has been recorded through the use of holography. Furthermore, this invention relates to an optical information recording and reproducing apparatus through the use of holography.
2. Description of the Related Art
Holographic recording means recording information in a recording medium through the use of holography. In general, the holographic recording is performed as follows. Information light which carries image information is generated. Reference light is also generated. The information light and the reference light are superimposed on each other in the recording medium, causing an interference pattern (a holographic pattern) therein. The interference pattern is written into the recording medium. The writing of the interference pattern involves the recording of the image information. The image information is reproduced from the recording medium as follows. The recording medium is irradiated with the reference light. The interference pattern in the recording medium diffracts the reference light. The resultant diffracted light carries the image information. The image information is extracted from the diffracted light.
In recent years, volume holography, or digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. The volume holography is a method of writing a three-dimensional interference pattern by making positive use of a recording medium in a direction of this thickness as well. The features of the volume holography are that the diffraction efficiency can be enhanced by increasing the thickness of the recording medium, and a greater recording capacity can be achieved by employing multiplex recording.
The digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns. In the digital volume holography, analog image information such as a picture is once digitized and developed into two-dimensional digital pattern information. Then, the digital pattern information is recorded in the recording medium as image information. For reproduction, the digital pattern information is read from the recording medium before being decoded to restore the original image information for display. Even if the signal-to-noise (the S/N ratio) during reproduction is poor, it is possible to reproduce the original information with extremely high fidelity by performing differential detection and employing an encoding decoding scheme inclusive of error correction.
Such two-dimensional digital pattern information recorded in a recording medium contains a sync code for a positioning purpose. A corresponding sync code is provided on an image detector including a CCD (charge coupled device) array or a COM (complementary metal oxide semiconductor) array. An image of the recording medium which carries the digital pattern information is captured by the image detector. Matching is taken between the sync code on the image detector and the sync code in the captured image. The matching means finding the position of the sync code in the captured image or making the two sync codes positionally coincide with each other. As a result of the matching, the positioning of the captured image is performed.
A typical example of two-dimensional digital pattern information contains a plurality of sync codes arranged at equal intervals in horizontal and vertical directions.
Japanese book entitled “Introduction to Computer Image Processing”, published in 1985, discloses template matching in which a small image is placed over a portion of a large image (a source image). The small image is referred to as a template which represents an object to be detected in the large image (the source image). During the template matching, the template is moved through different positions relative to the large image. For each of the different positions, the similarity is calculated between the template and the portion of the large image which overlaps the template. The maximum similarity is selected from the calculated similarities corresponding to the respective different positions. One of the different positions which corresponds to the maximum similarity is used as an indication of the position of the object to be detected in the large image. Alternatively, one of the calculated similarities which exceeds a threshold value may be found. In this case, one of the different positions which corresponds to the found similarity is used as an indication of the position of the object to be detected in the large image. The calculated similarities are expressed by, for example, calculated correlation values. The maximum similarity corresponds to the maximum one of the correlation values.
The template matching can be utilized for taking the previously-mentioned matching between the sync code on the image detector and the sync code in the captured image. The two-dimensional digital pattern information recorded in the recording medium is referred to as page data. In the template matching, a reproduced image obtained through the image detector and representing the page data is used as a source image while the sync code on the image detector is defined as a template representing an object to be detected in the source image. During the template matching, the template is moved through different positions relative to the source image. For each of the different positions, calculation is made as to the value of a correction between the template and the portion of the large image which overlaps the template. The maximum correlation value is selected from the calculated correlation values corresponding to the respective different positions. One of the different positions which corresponds to the maximum correlation value is used as an indication of the position of the object (the sync code) to be detected in the source image. The calculation of the correlation value for each of the different positions is implemented according to a prescribed equation which requires vast numbers of calculation steps. Accordingly, there occur drawbacks as follows. Hardware for implementing the template matching tends to be complicated in structure. In addition, the implementation of the template matching tends to take a long time.
Japanese patent application publication number 2004-310957 discloses an optical information reproducing apparatus designed to remove the above-mentioned drawbacks. The apparatus of Japanese application 2004-310957 serves to optically reproduce, from an optical information recording medium, information which has been recorded through the use of holography. The optical information recording medium has a recording layer holding the recorded information. The recorded information includes a sync code composed of pixels having a same value.
The apparatus of Japanese application 2004-310957 includes a light source for generating a light beam, a device for generating a reproduction-purpose reference light from the light beam generated by the light source, and a device for applying the reference light to the recording layer in the optical information recording medium. Reproduction light occurs as the reference light is applied to the recording layer. The reproduction light contains a reproduced image representing the recorded information. The apparatus further includes a device for collecting the reproduction light, and a detector for the collected light. The detector obtains the reproduced image from the collected light. The detector is provided with a sync code for positioning the reproduced image.
The apparatus of Japanese application 2004-310957 carries out improved template matching as follows. The sync code in the detector is composed of pixels having a same value. The sync code is used as a template while the reproduced image is used as a source image. The template is placed over a portion of the reproduced image (the source image). The template is moved through different positions relative to the reproduced image. The different positions are in a two-dimensional array. Specifically, the template is moved in an XY plane (a two-dimensional plane) of the reproduced image. First, the template is moved along only an X direction while the value of a correlation between the template and the portion of the reproduced image which overlaps the template is calculated for each of positions in an X-direction row of the array of the different positions. Second, the template is repetitively moved along a Y direction while the value of a correlation between the template and the portion of the reproduced image which overlaps the template is calculated for each of the different positions except the above-mentioned positions in the X-direction row. The maximum correlation value is selected from the calculated correlation values. One of all the different positions which corresponds to the maximum correlation value is used as an indication of the best positional matching between the template and the reproduced image.
The template matching in the apparatus of Japanese application 2004-310957 requires a less number of calculation steps due to the following factors. A sync code is composed of pixels having a same value. During the calculation of correlation values for the different positions, the template is moved along only an X direction before being repetitively moved along a Y direction.
A typical example of an optical information recording medium is an optical disc. An unevenness in the thickness of an optical disc, an irregularity of the plane thereof, and an eccentricity thereof result in a rotation, a curve, and a distortion of data reproduced from the optical disc.
The apparatus of Japanese application 2004-310957 can not deal with such a rotation, a curve, and a distortion of a two-dimensional digital pattern information (page data) reproduced from an optical information recording medium. Therefore, it tends to be difficult to detect the correct positions of bits constituting the two-dimensional digital pattern information.